In sulfite pulping processes, the cellulose fibers are liberated from the wood by dissolving lignin and some of the carbohydrate material. Since the principal constituents of the wood, i.e., cellulose, hemicellulose, and lignin, are chemically combined, the pulping process involves two principal types of concurrent reactions: the reaction of the lignin with the bisulfite, and the hydrolytic splitting of the cellulose-lignin complex. The mechanism of the removal of the sulfonated lignin has not yet been satisfactorily explained, perhaps because of incomplete knowledge of the structure of the lignin itself. Whatever the mechanism, the calcium salt of the sulfonated lignin (in the case of calcium-base liquors) is rendered soluble at the temperatures employed in the pulping process. The easily hydrolyzable hemicelluloses are dissolved at the same time. Fortunately, the major portion of the cellulose is comparatively stable under the processing conditions used.
An important requisite for wood to be used in the sulfite process is freedom from excessive amounts of resin. It is also important that any such resin in the wood be evenly distributed, for if it is localized at certain points, the wood at those places will remain hard and will cause shives in the pulp. Thus, the species which are most commonly used for sulfite pulp are spruce, balsam, hemlock, and the white or true firs. Though most hardwoods are readily reduced, their use has previously been limited due to their short fibers and the relatively low strength of the resulting pulps.
In sulfite processing, the liquor has less solvent power than the alkali used in the soda process, and any bark, decayed portions, or knots which go into the digester appear as dirt in the finished pulp. For pulp to be used as unbleached fiber, this also applies to the light-colored inner bark, since it changes color and shows as dark fibers in the pulp. For bleached pulp, this inner bark is harmless, since it bleaches quite readily.
Accordingly, unbleached sulfite pulp is not utilized in many applications for these reasons: Kraft pulp being preferred due to its higher strength and lower tendency to contain dark colored dirt particles.
Uniformity and cleanliness of chips are essential to clean pulp and obtain good yield. For best results, all chips cooked in the same charge are usually of one kind of wood and as nearly as possible of the same age and moisture content. This is because the moisture content of the chips can vary widely, depending upon the seasoning. Also, wet chips tend to pack more and cook more slowly than dry chips and may sometimes add to the difficulty in maintaining the acid strength during warm weather. Furthermore, some woods could not be cooked at all if charged dry, but would cook fairly easily if saturated with water by preliminary boiling and cooling before charging.
Sulfite pulp is generally processed in a digester as described in the following paragraphs. The general form of a sulfite digester is a vertical cylinder with conical or dome-shaped top and conical bottom, with the total length being about three times the diameter. Digesters of other forms are, of course, found in some of the older mills and horizontal digesters are sometimes used.
The method of making a cook depends on the type of process being employed. In one standard process, horizontal or vertical stationary digesters may be used, and all cooking is done by steam located in the bottom of the digester. The standard procedure is to fill the digester with chips and then steam gently for several hours with direct steam. After steaming, cold liquor is rapidly introduced. The temperature is then raised to about 110.degree. C. as rapidly as possible, although this may require as much as 12 hours due to the size of the digester. The temperature in the digester is gradually raised to about 120.degree. C., which is maintained throughout the cooking period. The sulfite liquor is then removed and the pulp is washed.
The liquor used in this process is about 3.5 to 4.5 percent total SO.sub.2 with 0.9 to 1.24 percent combined. The actual cooking time varies in different mills, usually varying from 20 to 30 hours, depending on the temperature employed. The particular advantages of this process are relatively strong fiber and high yield because of the comparatively weak acid and the relatively low temperature of cooking.
An alternate cooking process, known as the quick-cook process, directly introduces steam into the digester. In this method, the digester is filled with wood chips, resulting in a rather loose packing. The settling of the chips during the first part of the cook suffices to cover them completely with liquor. Mechanical chips distributors, which effect a closer packing, are often used to increase the amount of charge and yield per digester. Charging the acid liquor into the bottom of the digester is the most generally satisfactory method of introduction. Steam is introduced through the bottom of the digester cone and, in many mills, through nozzles near the top of the cone directed upward to provide a better distribution.
In steaming a cook, it is important to slowly increase the steam pressure in order to avoid attaining a high temperature before the liquor has had time to penetrate the chips, because this causes the chip centers to become hard and red or brown in color. Generally, the digester should be heated rapidly to the critical temperature (e.g., about 110.degree. C. for spruce and 120.degree. C. for hemlock), the rate of temperature rise being limited by the time necessary for thorough penetration of the chips by the liquor.
The influence of increasing the concentration of combined SO.sub.2 in the cooking liquor is mainly in the improvement of the yield and strength of the pulp. Concentrations are usually maintained at about 1.1 percent. Lowering the cooking temperatures tends to increase the time required for the same degree of purification but also tends to increase the yield and the uniformity of the pulp.
The treatment of the pulp after washing is largely a mechanical one, to remove dirt, knots, slivers, and uncooked or partly cooked chips, by means of screens or the like. Screening removes about 3 to 8 percent of the pulp produced. During these mechanical purification processes, the stock is diluted.
The chemical composition of unbleached sulfite pulp depends upon the degree of delignification or amount of cooking, the species of wood, and to some extent upon variations in the wood of any one species. As the cooking proceeds up to the point of exhaustion of the bisulfite, reductions occur in the yield, in the lignin content of the pulp, and in its chlorine consumption or bleach requirement. At the same time the alpha-cellulose content of the pulp is increased and its viscosity in cuprammonium solution is lowered. When strong pulp is desired for use in the unbleached condition, it is cooked less than for the more readily bleachable grades. In making pulp which is to be given further purification and bleaching treatment for the preparation of rayon and other cellulose derivatives, it is usually advantageous to accomplish as much purification as possible in the cooking treatment.
As noted previously, conventional sulfite pulp is washed to remove liquor, screened to remove long, wide fiber bundles and cleaned to remove short, choppy fiber bundles. This pulp is then deckered (removal of water) prior to storage or use in the mill. Despite these treatments, sulfite pulp contains many large pieces of dark colored bark particles and dark colored fiber particles which give the sheet a poor appearance. In addition to the dirt problem, sulfite pulp is known to have strength properties (especially tear) which are inferior to Kraft pulp.
Previously, those skilled in the art have pressed sulfite pulp in an attempt to improve the tear strength. They have also refined sulfite pulp atmospherically in an attempt to improve cleanliness with very minimal success. This refining step was usually carried out with the pulp at a low consistency and thus caused a substantial strength loss of the product with very little improvement in cleanliness. Atmospheric refining at high consistency resulted a modest improvement in cleanliness.
The inventors have now discovered improvements in the processing of sulfite pulp which render the dirt particles sufficiently small so that they are no longer objectionable from an aesthetic viewpoint, and which also enhance the tear and stretch properties of the pulp to levels approaching those of Kraft pulps, while maintaining breaking lengths above 5500 meters.